The use of LEDs in lighting applications is rapidly expanding, as they are much more efficient and have much longer life time than incandescent lamps. Compared with fluorescent lighting systems, LEDs also have an advantage that they do not contain harmful substances such as mercury.
LEDs are primarily DC devices and usually require a much lower voltage than the mains AC power provided by utility companies. For this reason, a power supply or “driver” is needed for converting the mains AC power to a DC voltage or current appropriate for driving the LEDs.
Conventional AC-DC converters can be used as LED drivers. They have been developed and perfected for many years. They provide high efficiency and can be manufactured in mass quantities with very low cost. However, they are fundamentally incompatible with conventional dimmers, such as TRIAC dimmers, that are designed primarily for incandescent lamps. The reason is conventional AC-DC converters try to, and in fact, have been perfected to, regulate the output voltage for a wide range of input voltage and load variations. Therefore, when a dimmer is used to reduce the voltage supplied to a conventional AC-DC converter, the converter still tries to maintain the same output voltage and will not dim the light, until the input voltage is so low that the converter cannot work anymore, it will abruptly shut off or become unstable.
Converters that are compatible with conventional dimmers (so called “dimmable LED drivers”) can be made using various techniques. Examples include designs described in U.S. Pat. Nos. 7,649,327, 7,852,017, 7,609,008, 7,038,399 and U.S. patent applications 20110043129, 20110037399, 20110012530, 20100295478, 20100277103, 20080278092, 20100134038, 20090295300. However, all these designs require significant deviations from the design of conventional AC-DC converters and some are so complex as to require the use of microprocessors. Therefore, these designs cannot leverage the well developed and perfected designs of conventional AC-DC converters. As a result, the currently commercially available dimmable LED drivers are usually much more expensive than conventional AC-DC converters of the same wattage. Furthermore, most of the commercially available dimmable LED drivers are constant current type while a constant voltage type of LED driver is more desirable especially in flexible LED strip applications.